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<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // var的使用
    // var message = '我是一个字符串'
    // var num = 123
    // var obj = { name: 'zhangsan' }
    // var arr = [1, 2, 3, 4]
    // console.log(typeof obj);

    // 变量声明提升
    console.log(a);
    {
      var a = 123
    }
    console.log(a);

  </script>
</body>

</html>